Polycrystalline silicon thin films on glass obtained by nickel-induced crystallization of amorphous silicon

J. A. SCHMIDT; N. BUDINI; R. D. ARCE; R. H. BUITRAGO

Utretch

Conferencia; 23rd International Conference on Amorphous and Nanocrystalline Semiconductors - ICANS 23; 2009

Hydrogenated amorphous silicon (a-Si:H) can be deposited on glass substrates at low temperatures with a high deposition speed, giving uniform films even over large areas. However, despite many years of research, the stability problems could not be overcome and the electrical transport properties are worse than those of crystalline silicon (c-Si). Recently, renewed interest arose in using a-Si:H as a base material to obtain thin polycrystalline silicon (pc-Si) films on glass substrates. Having a grain size over 10 μm and an intra-grain quality comparable to that of c-Si, pc-Si is an attractive material to produce active-matrix liquid crystal displays, active-matrix organic light emitting diodes, and solar cells. In this work, we explore the nickel-induced crystallization (NIC) of a-Si:H thin films suitable for the production of pc-Si solar cells. The NIC process allows to crystallize a-Si:H films at temperatures compatible with the utilization of glass substrates. We used the plasma-enhanced chemical vapor deposition method to deposit a-Si:H films on planar float glass (Schott AF37). The films, between 100 and 400 nm thick, were grown intrinsic or slightly p-type (p-). On these films we sputtered nickel with concentrations between 1.5x10^13 and 3x10^14 at./cm^2, and then we annealed the samples in a standard nitrogen-purged tube furnace. The annealing sequence started with 12 h at 400 ºC to de-hydrogenate the samples, and then 24 h at 570 ºC to achieve crystallization. The process evolves through the formation of the nickel silicide NiSi2, which has a lattice constant very similar to that of c-Si and acts as a nucleation centre. As a result of this thermal treatment we obtained thin polycrystalline films with a grain size over 100 μm, which is a promising result. The high crystallinity of the samples was confirmed through optical and electron microscopy observations, X-ray diffraction and reflectance in the UV region. To approach the structure of a solar cell, we deposited p+/p- layers with thicknesses 80 nm/320 nm, where p+ is a highly boron doped layer. Applying the same nickel concentrations as before, full crystallization was reached at 550 ºC – a lower temperature than in the previous case. However, the grain sizes that we obtained were between 50 and 70 μm. This is an indication that high boron concentrations favor nucleation, decreasing the grain size. Using these crystallized p+/p- structures as seed layers, we deposited additional p- a-Si:H layers with a thickness of 1 μm. Then we annealed these samples at 400 ºC for 24 h for de-hydrogenation and at 570 ºC for 24 h for crystallization. The aim of this annealing was to achieve a solid phase epitaxy process, where the crystallizing a-Si overlayer copies the crystalline structure of the seed layer. We succeeded in this goal, obtaining pc-Si films with a total thickness of ~ 1.4 μm and with grains as large as 70 μm. Therefore, by depositing a thin amorphous or microcrystalline n+ layer on this base structure, a thin film silicon solar cell can be obtained.